1. Field of the Invention
The present invention generally relates to hologram scales and, more particularly, is directed to a hologram scale suitably applied to a position detecting device of an absolute system which detects, for example, a relative displacement amount between two members which are relatively displaced from each other as an absolute position from an origin on the scale.
2. Description of the Related Art
Generally, a conventional hologram scale is utilized as a so-called incremental scale. When this incremental scale is in use, a hologram scale on which a scale is formed by hologram gratings is located to one member and a detector for generating a pulse corresponding to the number of gratings by reading the scale is located to the other member which is relatively displaced from one member. Then, the relative displacement amount between one and the other members can be measured by adding the number of pulses.
However, since the conventional hologram scale is utilized as the incremental scale as described above, the origin of the coordinate system must be set again after the measuring work is interrupted and the power switch of the hologram scale is turned off or after the occurrence of power failure. Further, if the calculated value of the pulses becomes erroneous due to a noise once, then the following calculated values all become erroneous, which requires the origin to be set again similarly as described above.